In electrorefining (ER) and electrowinning (EW) electrodes are immersed in an electrolyte and an electric current is passed between them. The anode is made positive and the cathode made negative so that an electric current passes through the electrolyte from anode to cathode.
In electrorefining (ER), the metal anode is soluble. That is to say that the metal enters into the electrolyte under the influence of the potential between the anode and cathode. For example, in the electrorefining of copper, the anode is made of impure metallic copper and copper ions enter the electrolyte from the anode. The copper ions, now in the electrolyte, are transported through or by the electrolyte to the cathode where they are deposited. The cathode may be of the same metal as the metal that is being deposited or it may be of a different metal. For example, in the electrorefining of copper it was at one time common to employ a cathode made of copper. However, a stainless steel permanent cathode is now commonly employed which quickly becomes coated with copper and which from then on essentially performs as a copper cathode. The deposited copper is mechanically removed or stripped from the permanent cathode and the permanent cathode reused. The copper deposited on the cathode is highly pure. Impurities that were in the impure anode may dissolve into the electrolyte or fall out as a solid as the anode is dissolved and may contain useful byproducts, for example, gold. Besides copper, metals purified by ER include gold, silver, lead, cobalt, nickel, tin and other metals.
Electrowinning (EW) differs from electrorefining in that the metal sought is imported into the cells and is already contained within the electrolyte. In the example of copper, sulphuric acid is typically employed to dissolve copper from an oxide form of copper ore and the resulting liquor, after concentration, is imported into an electrowinning cell to have the copper extracted. An anode and cathode are immersed in the electrolyte and a current is passed between them, again with the anode being positive and the cathode being negative. In electrowinning, the anode is not soluble but is made of an inert material. Typically a lead alloy anode is used in the case of copper electrowinning. The cathode may be of the same metal that is being extracted from the electrolyte or it may be of a different material. For example, in the case of copper, copper cathodes may be used although stainless steel cathodes are commonly employed which quickly become coated in copper. Under of the influence of an electric current, the metal to be won leaves the electrolyte solution and is deposited in a very pure form on the cathode. The electrolyte is circulated and concentrated by this process having given up a large proportion of its metal content. Besides copper, metals obtained by electrowinning include lead, gold, silver, zinc, chromium, cobalt, manganese, aluminium and other metals. For some metals, such as aluminium, the electrolyte is a molten material rather than an aqueous solution.
As an example of the voltages and current involved, in copper refining, the cell voltage is generally about 0.3V and in copper electrowinning is about 2.0V. In both cases the cathodic current density is about 300 A/m2 and the area of each side of the electrode at present is about 1 m2. These figures differ considerably for different metals and widely varying current densities may be used for the same metal but the invention applies to the electrorefining and electrowinning of all metals.
In ER and EW the starting point is an anode juxtaposed to a cathode in an electrolyte contained in a tank. But many cathode plates and many anode plates may be used, interleaved, with all the anode plates connected in parallel and all the cathode plates connected in parallel contained within a single tank of electrolyte. Electrically this still looks like a single cell and in the industry it is therefore commonly called a cell. In the ER and EW industry, “cell” is almost universally used to mean a tank filled with anodes and cathodes in parallel. In the ER and EW industry, “tank” can mean the same as “cell”, above, or it can mean the vessel alone, depending on the context. In tankhouses cells are connected electrically in series. A typical ER tankhouse might therefore require an electrical supply of the order of 36,000 Amps at 200 Volts.
The electrical circuit representing a typical tankhouse is shown in FIG. 1. Tanks 3, each containing one cell (composed of many cathodes 1 in parallel and many anodes 2 in parallel), are connected in series. A DC voltage source 27 is connected across the series circuit to drive the desired current through the cells 3. The total current is maintained at a desired value. Ideally, the current should divide equally between the cathodes 1. In practice, there is significant variation in the resistance of each cathode-anode current path and hence there are variations in the values of the individual cathode currents. This means in practice that the metal production process operates at below optimum efficiency.
More seriously, there is sometimes disruption to the operation of part of the cell when a short circuit develops between an anode plate and a cathode plate. This is typically due to a nodule or dendrite of metal growing from a cathode plate and increasing in size until it connects with the adjacent anode plate. The nodule of metal has to be physically removed to permit normal operation to continue.
Another disruption to normal production can occur when an individual cathode or individual anode becomes disconnected from the electrical circuit. As FIG. 2 shows, the electrical connection to cathodes 1 and to anodes 2 is typically made through lugs or hanger bars 13 which project from each side of the electrodes. On the right side, the hanger bar 13 rests on a busbar arrangement 4 which forms part of the electrical circuit. The disconnection is typically caused by corrosion or burning of the contact point 12 or by a foreign obstacle becoming jammed between the hanger bar 13 and the busbar 4 or build-up of sulfate between the hanger bar 13 and the busbar 4. On the left side, the other hanger bar 13′ may either rest on an insulated supporting bar 4′ or this bar may be a secondary busbar, also known as an equaliser bar, so that the electrode 1 is electrically connected through two paths so as to reduce the effect of a bad contact to one of the hanger bars 13.
A short circuit results in an unusually large amount of current flowing in the cathode 1 and the anode 2 which are electrically shorted together. Methods conventionally employed to detect short circuits are less than ideal. One method is to detect the overheating of the electrodes resulting from the short circuit. This is less than satisfactory because damage to the electrode, its hanger bars 13 or the busbar 4 may have resuited due to a time delay before the short is detected.
This method will become even less acceptable as new, expensive, high-performance anodes, are introduced into electrowinning processes. In electrowinning, inert lead anodes have been commonly used. In recent years mixed metal oxide (MMO) catalytically coated titanium anodes have been increasingly adopted because of their superior properties. However, the MMO coated titanium anodes are more expensive than lead based anodes and more easily damaged by the heat generated during shorting. It has therefore become imperative that problems with the process, in particular short circuits between electrodes, are identified very quickly. Furthermore it is desirable that circumstances likely to give rise to a short circuit are identified. One indicator of an incipient short circuit is a rise in cathode or anode current above its usual value. Hence current measurement with an accuracy and resolution suitable for detecting this rise in current is a tool for identifying dangerous situations and for prompting operator action to correct the situation.
Another method of detecting shorts is to have a worker patrol the tanks using a Gaussmeter to detect the high magnetic field produced by the short-circuit current. Due to restricted labor the patrol can often be organized only once per day or a maximum of few times per day. Therefore the short may go undetected for many hours, during which time production is lost, current efficiency decreases, the risk of decreased cathode quality increases and the electrodes, hanger bars and busbars may be damaged. This method has also proved very inefficient because the patrol needs to check every cell including the cells that do not have any problems. Unnecessary walking on top the cells during the patrol may also cause electrode movement and thus new short circuits. It also increases the risk of accidents. Infrared cameras are also used either by the worker patrols or in overhead cranes to detect short circuits due to heat caused by high current. The method has often proved not to give the desired results in the tankhouse environment because of the long time delay in detecting a short and also availability issues of the crane for the monitoring task.
In order to detect short circuits and bad (open) contacts there is a need to detect these problems at the level of individual cathodes or anodes by providing methods for measuring the current flowing in individual electrodes.
In prior art, U.S. Pat. No. 7,445,696 discloses an electrolytic cell current monitoring device and method, which detects not only short circuits, but open circuits as well. The apparatus comprises magnetic field sensors, e.g. Hall effect sensors, that measure magnetic field strength generated around a conductor adapted to carry electrical current to or from an electrolytic cell. The magnetic field current sensors for each cathode may be arranged on a rail car device which operates above the cells to detect the shorts and open contacts. Detection of current in all cathodes in the cell can be made simultaneously. The magnetic field sensor is brought at a distance above each electrode hanger bar aided by a capacitive proximity sensor.
Further prior art is disclosed in an article “Measurement of Cathodic Currents in Equipotential Inter-Cell Bars for Copper Electrowinning and Electrorefining Plants”. Industry Applications Conference, 2007. 42nd IAS Annual Meeting. Conference Record of the 2007 IEEE; 23-27 Sep. 2007; Wiechmann, E. P., Morales, A. S.; Aqueveque, P. E.; Burgos, R. P. pp. 2074-2079, proposes a measurement technique for the cathodic currents in a dog-bone type intercell bars using linear ratiometric Hall effect sensors and ferromagnetic flux concentrators. The article discloses that cathode currents may be measured by combining the magnetic flux sensors and flux concentrators.
Prior art solutions which are arranged to detect adverse conditions in the cells has focused on measuring the current flowing into or out of cathodes or anodes. The path which these currents take is through a hanger bar or lug, through a contact point between the hanger bar and busbar and then through a busbar. The path is one which changes direction or plane at least twice and after passing through the contact point its exact path is uncertain and variable. Hence the electrode currents are only realistically measurable in the hanger bars or at the contact point.
However, the measurement of the current in the hanger bars or at the contact point alone does not give the tankhouse operators sufficient information of the amount of current which actually flows in the electrode in the event there is a bad or open contact between the hanger bar of said electrode and the electric busbar due to that the equaliser busbar, which supports the other end of the electrode on the other side of the cell, is able to provide multiple pathways for the current so that current is conducted via the equaliser busbar from the adjacent other electrodes to the electrode which has the bad or open contact. Under ideal operating circumstances the equaliser bars connect points in the system which are at the same potential and the equaliser bar carries no current. When conditions are non-ideal, due to a variety of possible adverse circumstances such as said bad or open contacts, currents flow in the equaliser bars which tend to restore the system closer to its ideal operating condition. Currents flowing in the equaliser bars are therefore an indication that problems are arising with the process. Without the knowledge of the actual current flowing in the electrode it is not possible to detect or predict short circuits and open and bad contacts with a great certainty. Therefore, in order to obtain accurate current measurements for each individual electrode there is still a need for a measurement of the current which flows in the equaliser busbar between each contact point.